There are various applications where it is desirable to pass light down an optical fibre and to emit the light substantially isotropically (that is, with equal energy density in all directions) from the remote end of the fibre, or to receive substantially isotropically at the remote end and transmit it along the fibre. One such application is in medical endoscopy, where it is desirable to be able to measure the intensity of light energy actually reaching a treatment site. This could best be done by a detector which is isotropic, which is sufficiently small and flexible to pass through an endoscope, and which is formed of clinically compatible materials.
It has been attempted to form an isotropic emitter/detector by bonding a sphere of a light-scattering material such as PTFE to the end face of an optical fibre. It is difficult to form a satisfactory bond, and there is a substantial risk of the bond failing in use. It is also difficult to machine the spherical member accurately, a typical diameter being about 1 mm.